1. Field of the Invention
The present invention generally relates to the field of semiconductor device fabrication, and more particular, to usage of deuterated materials in semiconductor device processing.
2. Description of the Related Art
Electronic devices, such as metal oxide semiconductor (MOS) transistors, used in VSLI integrated circuits experience a number of wear out mechanisms that limit the degree to which they can be miniaturized (scaled). One of these mechanisms is the so-called hot (energetic) electron effect.
For instance, in thermally oxidized silicon, such as gate oxide formed on a silicon substrate, it is possible for electrons (or holes) generated in the silicon substrate by device action to escape from the silicon and become injected into and be trapped in the adjoining silicon oxide. Depending on the use conditions and the details of the structure of the source and drain diffusions of a MOSFET device, there will be more or fewer electrons of greater or lesser energy generated in the silicon substrate and injected into the gate oxide.
The trend in VSLI device design has been towards increased electric field (both vertical and horizontal), which aggravates hot electron effects. Among other things, the hot electron effects result in a slow, long-term change in the threshold voltage, as well as a reduction in the transconductance of MOSFET devices. Hot electron effects also can give rise to the known phenomenon of deterioration in the low-level current gain of bipolar transistors whose emitter-base junctions have been subjected to avalanche breakdown.
The accepted theory on how the hot electrons cause damage to the silicon/silicon oxide interface is that the hot electrons stimulate desorption of hydrogen from the silicon/silicon oxide interface by breaking some of the Si--H bonds present on the silicon surface, leading to an increase in interface trap density and a degradation in device performance. The hydrogen is present at the interface after being introduced into the device as result of semiconductor processes such as post-metallization anneals of wafers conducted at low temperatures in hydrogen ambient, which improve device function by passivation of crystal defects at the silicon/silicon oxide interface. The term "passivation" means that the hydrogen satisfies dangling bonds at the silicon/silicon oxide interface. However, the Si--H passivation bonds formed at the silicon/silicon oxide interface during such annealing processes are susceptible to dissociation by hot electron excitation.
It has recently been found that the hot electron effect may be mitigated by replacing deuterium (D) for hydrogen used in the passivation of the interface traps at the silicon/silicon dioxide interface. (J. W. Lyding et al., Appl. Phys. Lett. 68 (18), Apr. 29, 1996, pp. 2526-2528; and I. C. Kizilyalli et al., IEEE Electron Device Letters, vol. 18, No. 3, March 1997, pp. 81-83.) The element hydrogen has three known isotopes:
ordinary hydrogen, or protium, .sup.1.sub.1 H; heavy hydrogen, or deuterium, .sup.2.sub.1 H; and tritium, .sup.3.sub.1 H. Lyding et al. and Kizilyalli et al. teach that the deuterium isotope accumulates at the silicon/silicon dioxide interface during post-metal anneal processes, such as is done in a deuterated forming gas (D.sub.2 /N.sub.2) at a temperature of 400.degree. C. The resulting silicon-deuterium (Si--D) bonds formed at the silicon/silicon dioxide interface are found to be more resistant to dissociation from hot electron excitation stresses than a Si--H bond.
The present investigators, however, have determined that deuterium incorporated into a silicon/silicon dioxide interface for passivation by anneal processes tends to drift away from the interface as a result of subsequent thermal cycling incurred in the further processing of the semiconductor device, even where only relatively moderate temperatures, such as about 400.degree. C., are involved. In particular, the present investigators have confirmed with secondary-ion-mass spectroscopy (SIMS) data that deuterium incorporated at the silicon/silicon dioxide interface can and does migrate away from the interface during subsequent processing of the sort which effectively "anneals" the wafer as the intended objective or as an incidental effect of a different process, such as film deposition. Therefore, any potential performance enhancements imparted by the deuterium anneal of the prior art were evanescent in nature based on what the present investigators have determined. For purposes of this application, the term "anneals", and variants thereof, means subjecting a semiconductor wafer to at least one thermal cycle in which it is heated and thereafter cooled.
Therefore, problems remained unresolved in the prior art with respect to semiconductor processing involving hydrogen-containing reactants and ambients which tended to cause silicon/silicon dioxide interface disruption.